There has been extensive independent development of vibration-based and photovoltaic-based passive energy harvesting; however, both approaches have their major limitations due to limited environmental condition operational windows that are appropriate for energy conversion to function.
Certain ferroelectric materials (e.g., quartz and Rochelle salts, and bulk ceramic materials) are known to produce a voltage between surfaces of a solid dielectric when a mechanical stress is applied. This phenomenon is known as the piezoelectric effect and may be used to convert mechanical energy, such as vibration, to electrical current. Such materials are now found in both stiff ceramic, soft polymeric and semi-flexible multi-laminates such as described in U.S. Pat. No. 6,665,917 entitled “METHOD OF FABRICATING A PLANAR PRE-STRESSED BIMORPH ACTUATOR” and issued to Knowles et al.
When subject to vibratory conditions such materials will yield electrical energy; however, in practice this is often not the situation. There may be periods for which little or no vibration is present, such as in storage or idle mode. An example is vehicular mounted vibration conversion devices. These can convert while the vehicle is in transit, but they do not generate electrical energy when the vehicle is stationary or moving slowly.
Harvesting solar or light energy is known. In theory, devices having solar cells never need batteries and can work forever. Photovoltaic cells or modules (a grouping of electrically connected cells) can be provided in a device to convert sunlight into energy for powering a device. An example of a self-powered solar system includes U.S. Pat. No. 6,914,411 entitled “POWER SUPPLY AND METHOD FOR CONTROLLING IT” and issued to Couch et al. Couch et al. discloses a self-powered apparatus including a solar power cell, a battery, and a load. The load may include one or more load functions performed using power provided by one or both of the solar power cell and the battery. Switching circuitry, controlled by the programmable controller, selectively couples one or both of the battery and the solar cell to supply energy for powering the load. In a preferred embodiment taught by Couch et al, the controller couples the battery and/or solar cell to charge a super capacitor.
However, photovoltaic-based passive energy harvesting devices are non-functional for significant periods of time over which the systems they are integrated into, or attached to, are not exposed to sunlight, resulting to this type of self-power as being unreliable. An example is roadway mounted photovoltaic panels. These are typically pole-mounted on the roadside as to power either traffic monitoring or remote communication capabilities. At night or during inclement weather such energy conversion mechanisms are non-effective in conversion of energy to electrical form. Another example is self-powered remote communications that come equipped with a solar panel to generate energy as to extend battery lifetime. These can convert sunlight into electrical energy using the solar photovoltaic energy harvesting effect, but there will be long periods over which such sunlight is absent.
However, it is interesting to observe that these same roadway solar panels can be subject to vibratory excitation during these non-functional solar periods due to a number of sources such as wind, passing traffic or heavy rain. Again, it is interesting to observe that remote deployed communications can be subject to vibratory excitation during these non-functional solar periods due to a number of sources such as wave motion, physical transportation or in situ motion—such as soldier movements.
Although it is feasible to install both photovoltaic energy conversion devices and vibratory conversion devices as to substantially broaden the overall window of energy harvest conditions, it is usually unrealistic for several reasons: increased complexity of having two separate systems incurring two sets of installation time, cost, and volume penalties; and two sets of electronics; form/fit/function limitations that can prevent multiple energy harvest systems within a single platform or application; system connector limitations and accumulation of system loss.
What is needed is a solution to enabling a single integrated device that is approximately the same physical size, manufacture cost and install complexity of a standard photovoltaic energy harvester, such as a solar panel or array, but that can simultaneously act as to efficiently convert mechanical energy even when there is no sunlight as to provide a more continuous source, or an enhanced source when sunlight is available, of passive environment energy conversion to electrical energy.